Pulmonary surfactant is essential for normal lung mechanics and gas exchange in the lung. Pulmonary surfactant is produced by type II epithelial cells and is made up of a phospholipid component which confers the ability of surfactant to lower surface tension in the lung. In addition, there are proteins associated with the surfactant called collectins which are collagenous, lectin domain-containing polypeptides. Two of these, designated surfactant protein A (SP-A) and surfactant protein D (SP-D), are likely involved in surfactant structure and function and host defense. Both quantitative and qualitative deficiencies in pulmonary surfactant are associated with neonatal respiratory distress, adult respiratory distress syndrome, congenital deficiencies of surfactant protein B, and allergic asthma. In addition, deficiency in pulmonary surfactant may contribute to the increased susceptibility of some individuals to microbial challenge, especially in the setting of inadequate or impaired specific immunity. These disorders as well as some disorders associated with increased risk of pneumonia (cystic fibrosis, asthma, prematurity, chronic bronchitis, diffuse alveolar damage) may also be associated with acquired defects or deficiency in collectin function. Alveolar surfactant pools are regulated at multiple levels including intracellular synthesis, secretion, re-uptake and degradation of these components by alveolar macrophages. The synthesis and clearance of surfactant phospholipids and proteins is further influenced by developmental, mechanical, and humoral stimuli that serve to maintain steady-state surfactant concentrations after birth.
The role of the collectins in surfactant and normal lung function has been extensively investigated. The collectin family of C-type lectins includes a number of molecules with known host defense functions. SP-A and SP-D, also C-type lectins, bind influenza and herpes simplex viruses as well as gram positive and gram-negative bacteria and various fungi. By binding they enhance uptake by alveolar macrophages and neutrophils. Various cellular binding sites for SP-A and SP-D have been identified on alveolar macrophages or, in the case of SP-A, on type II epithelial cells. The critical role of SP-A in host defense was supported by the observation that SP-A-deficient mice are susceptible to infections by group B streptococcus, Pseudomonas aeruginosa, Respiratory syncytial virus, adenovirus, and mycoplasma in vivo. Thus, there is a clear role for SP-A and a likely role for SP-D in respiratory defense mechanisms. Collectins may also participate in the recognition or clearance of other complex organic materials, such as pollens and dust mite allergens. However, to date no human diseases have been associated with specific deficiencies in SP-A or SP-D.
SP-D is a 43 kilodalton protein that has been proposed to play a role in host defense in the lung. Its cDNA and gene have been sequenced in various mammals including humans. SP-D shares considerable structural homology with other C-type lectins, including surfactant protein A (SP-A), conglutinin, bovine collectin-43, and mannose binding protein. In vitro studies and its close structural relationship to a mammalian Ca2+-dependent lectin family (particularly shared structural motifs) support its role in host defense. SP-D is synthesized primarily and at relatively high concentrations by Type II epithelial cells and nonciliated bronchiolar epithelial cells in the lung but may also be expressed in the gastrointestinal tract, heart, kidney, pancreas, genitourinary tract and mesentery cells. In vitro studies demonstrated that SP-D binds to the surface of organisms via its lectin domain (or sugar binding domain) which leads to binding, aggregation, opsonization and, in some instances, activation of killing by phagocytes in vitro. SP-D binds to lipopolysaccharide, various bacteria, fungi and viruses, including influenza virus. It also binds to both alveolar macrophages and polymorphonuclear cells. It may possibly play a role in surfactant phospholipid homeostasis, including the effects of SP-A on phospholipid metabolism by Type II cells in vitro, however, this is controversial and the precise role of SP-D in vivo is still unclear.
In vitro studies support the concept that surfactant proteins may be important in the regulation of surfactant homeostasis. Although the hydrophobic surfactant proteins SP-B and SP-C have roles in production of the surfactant monolayer, in vitro studies indicated that surfactant protein A may also facilitate surfactant uptake and/or secretion by type II epithelial cells. In fact, it was widely believed that SP-A would have a major role in surfactant homeostasis. However, recent studies of SP-A null mice have not supported the primary role of surfactant protein A in surfactant secretion or re-uptake. The absence of SP-A does not lead to obvious physiologic or morphologic structural abnormalities of the lung. SP-A null mutant mice lack tubular myelin figures but produce highly functional surfactant that absorbs rapidly and produces monolayers. Surfactant lipid synthesis, secretion, and re-uptake were essentially normal in SP-A null mice.
Therefore, the additional surfactant protein which acts in surfactant regulation is still not identified. In addition, the precise role of SP-D in normal lung function has not been clearly defined at this point and its role in disease or disease susceptibility is unclear.